274 PRACTICAL ORGANIC CHEMISTRY PREPARATION 48 Nitrobenzene.—The formation of nitro-compounds, by the action of strong nitric acid on the hydrocarbon, is a distinctive property of aromatic compounds, although recent researches have shown that dilute nitric acid under pressure will convert some of the paraffins, especially the tertiary hydrocarbons, into mono- and cli-nitro-derivatives. The production of nitro-com- pounds is usually effected by strong or fuming nitric acid, or solid potassium nitrate, in presence of cone, sulphuric acid. Where the action is vigorous, as in the case of the phenols, it is necessary to use moderately dilute acid. The number of hydrogen atoms replaceable by the nitro-group (N02) is limited. In benzene the first nitro-group is introduced with great ease, the second less readily, and the third with some difficulty. The position taken up by the nitro-groups may be briefly stated as follows : When a negative group (nitro, carboxyl, cyanogen, aldehyde) is already present, the nitro-group enters the meta- position to the first group. In the presence of other groups (alkyl, hydroxyl, halogen, ammo), the nitro-group attaches itself to both ortho- and para-positions. Benzoic acid and benz- aldehyde give, on nitration., mainly meta-compounds, whereas toluene, phenol, and aniline form simultaneously ortho- and para-derivatives. Nitro-compounds have often a yellow or red colour, are with difficulty or not at all volatile, possess a much higher boiling- point than the corresponding halogen derivatives, and are denser than water, and insoluble in that liquid. PREPARATIONS 49—51. Azoxybenzene, Azobenzene, Hydrazobenzene.—- Nitro-compounds yield a series of reduction products accordingto the nature of the reducing agent. Alkaline reducing agents : sodium methylate, zinc dust and caustic soda, stannous chloride and caustic soda, produce azoxy, azo- and hydrazo-compounds. C6H5N02 C6H5N. C6H5N C6H5NII I >0 || | C6H8N02 C6H5N/ C6H3N C(5H5NH Nitrobenzene. Azoxybenzene. Azobenzene. Hydrazobenzene. The sodium methylate acts as a reducing agent by taking up oxygen and forming sodium formate.